Hydrophilic reflective article

ABSTRACT

A composite article, in particular, a rear-view mirror for a motor vehicle, comprising a substrate generally made of glass, a reflective layer composed of an oxidized or nitrided metal, possibly in an under-stoichiometric state disposed on either the front face or the rear face, and a coating stack disposed on the front face. The coating stack comprises a layer having generally photocatalytic properties, preferably a titanium dioxide based layer, which may be covered by a fine hydrophilic layer. The coating stack may also comprise a barrier sub-layer. Preferably the hydrophilic layer and barrier sub-layer are composed of silicon oxide. A process for forming the composite article includes sputter deposition of various layers followed by heat treatment. Advantageously the article has an attenuated reflection which can vary between 40% and 75%.

The present invention relates to a reflective article, in particular forthe rear-view mirrors for motor vehicles, having hydrophilic propertiesand an attenuated reflection factor. The present invention also relatesto a process for the production of such an article.

Mirrors comprising a metal layer (generally made of silver, aluminium orchromium) applied either to the rear face of a transparent substrate,i.e. on the face remote from the observer, or on the front face of thesubstrate, thus the face directed towards the observer, are known. Witha metal layer of chromium having a thickness in the order of 40 to 60nm, a light reflection of about 65% is obtained, which is perfectlysatisfactory for use as a rear-view mirror. However, with moresignificant light reflections the rear-view mirror has the disadvantageof causing glare for the driver.

Mirrors with a surface, which has been rendered hydrophilic, are alsoknown (see EP 689 962, EP 1 022 588 or JP 2001033607, for example).

The hydrophilic character of a surface increases its surface energy,which allows drops of water to spread in a film instead of formingdroplets. On a non-hydrophilic mirror the rain forms droplets, whichobstruct visibility. On a mirror with a hydrophilic surface, the waterspreads to form a film to allow better visibility. Various materials areknown for their inherent hydrophilic properties, in particular titaniumoxide and silicon oxide.

In addition to its hydrophilic properties, titanium oxide, particularlywhen crystallised in the form of anatase, is also well known for itsinherent photocatalytic properties, i.e. it is able to degrade organicmatter when stimulated by light or UV irradiation.

Patent applications EP 978 494 and EP 1 099 671 describe anti-fogmirrors comprising a reflective metal film respectively on the rear andfront face and a TiO₂/SiO₂ coating stack on the front face.

Since the TiO₂ layer has a high refractive index (n=2.4), the reflectionfactor of the coating stack in the visible range is elevated, in theorder to 80% for a stack of neutral colouration. To reduce glare, thethicknesses of the layers must be selected so that the wavelength of thereflected light has a peak between 400 and 510 nm, which gives areflected blue colour and a light reflection in the order of 60%. EP 1099 671 provides that a reflection-adjusting layer can be added betweenthe reflective film and the TiO₂ layer to prevent excessive reduction ofthe light reflection.

Coating stacks with alternating layers of high and low refractive indexare commonly used to increase the light reflection. Documents EP 456 488and EP 1 040 963 describe mirrors with high light reflection (>70%)using a metal layer as reflective layer, and a succession of low indexlayers (SiO₂) and high index layers (TiO₂) to increase reflection.

There is a need to provide a reflective article with a photocatalyticand hydrophilic effect to allow good visibility in the case of rain,while maintaining a moderate reflection factor to decrease glare. Itmust be possible in a simple manner to provide such an article withmoderate reflection in neutral reflected tones as well as in colouredtones, e.g. in the blue range.

The aim of the invention is to remedy the disadvantages described above.In particular, an aim of the present invention is to provide areflective article, which has hydrophilic and photocatalytic properties,and a light reflection, which is maintained at a level of reflectionthat is not excessive even with a neutral colouration.

The subject of the present invention is a composite article comprising asubstrate, a reflective layer (generally referred to as reflector)composed of an oxidised or nitrided metal, possibly in anunder-stoichiometric state, possibly covered by a barrier layer, then atitanium dioxide-based layer with photocatalytic properties, thenpossibly a fine porous hydrophilic layer composed in particular ofsilicon oxide. This surface layer can be discontinuous.

In particular, the reflective layer is a Cr_(x)N_(y) wherein x iscomprised between 0.67 and 0.9, preferably between 0.7-0.8 and y iscomprised between 0.1-0.33, preferably between 0.2-0.3.

According to an advantageous embodiment shown in FIG. 1, the layers aredisposed on the same face of the substrate. However, it is also possibleto dispose the reflective layer on the rear face, i.e. on the faceremote from the observer, and the photocatalytic layer on the frontface, as shown in FIG. 2.

The thickness of the photocatalytic layer can be in the range of between20 and 120 nm and preferably between 40 and 75 nm. This thickness of thesurface layer is itself generally in the range of between 2 and 10 nmand preferably between 3 and 8 nm. This latter layer allows thehydrophilic character of the surface to be preserved for longer afterthe light irradiation has ceased. The very fine thickness of this outerlayer enables the photocatalytic effect of the TiO₂ layer to bepreserved to some extent.

When a barrier layer is disposed between the reflective layer and thephotocatalytic layer, this barrier layer is advantageously composed ofsilicon oxide. Its thickness can lie between 10 and 80 nm and preferablybetween 20 and 60 nm. As a result of this barrier layer, the migrationof alkaline constituents of the glass, in particular Na⁺ ions, towardsthe titanium oxide layer can be reduced or prevented, and also thetitanium oxide layer can be separated from the reflector.

The metal of the reflective layer can be selected from titanium,chromium, aluminium, silicon, zirconium and alloys of these metals.Advantageously, the reflective layer is composed of partially oxidisedor nitrided chromium. Its thickness can lie between 20 and 150 nm,preferably between 40 and 120 nm.

It is advantageous if the above-described reflective article has a lightreflection (integrated over the entire visible range) in the range ofbetween 40 and 75% and preferably between 45 and 70% of the incidentvisible light.

When the reflected colour of the article according to the invention isneutral (i.e. when the coefficients a* and b* of the Lab system liebetween −5 and 5), it is advantageous if the reflection factor liesbetween 55 and 75%, preferably between 60 and 70%, and when thereflected colour is within the blue range (i.e. a* lies between −10 and0 and b* is less than −10), it is advantageous if the reflection factorlies between 40 and 55%, preferably between 40 and 50%. The coefficientsa* and b* are measured with the illuminant D65 at an angle of incidenceof 2°.

The light transmission of the article must be very low and preferablyless than 3%, indeed less than 2%.

The present invention also relates to a process for the production of areflective and hydrophilic article, which comprises the following steps:

the deposit of a lightly oxidised or nitrided metal layer (20) on thefront or rear face of a support by cathodic magnetron sputtering in acontrolled reactive atmosphere with a metal target;

possibly the deposit of an SiO₂ barrier layer on the front face of thesupport by cathodic magnetron sputtering in a reactive atmosphere withan Si target;

the deposit of a TiO₂ layer on the front face by cathodic magnetronsputtering, e.g. in a reactive atmosphere with a Ti target;

thermal treatment at a temperature in the range of between 300 and 500°C., In particular between 350 and 450° C., for a period that may varyfrom 15 minutes to 6 hours, in particular from 30 minutes to 4 hours,which allows the TiO₂ to crystallise in the form of anatase whilepreventing crazing of the TiO₂ and the haze which would resulttherefrom.

In particular, a process according to the invention also comprises astep of depositing a fine surface layer of SiO₂ by magnetron sputteringin a reactive atmosphere with an Si target.

When the reflective layer is disposed on the rear face, this isadvantageously deposited first. The barrier and photocatalytic layersand the surface layer are then deposited on the opposite face. Theentire covered substrate can then be subjected to a thermal treatment.

The present invention is described below by non-restrictive practicalexamples.

EXAMPLE 1

A coating stack comprising glass/Cr_(x)O_(y)/SiO₂/TiO₂/SiO₂ of neutralcolouration, as shown in FIG. 1, is formed on a clear soda-lime glass(10) with a thickness of 2 mm by cathodic magnetron sputtering.

The conditions of depositing the different layers forming the stack areas follows:

A first layer (20) of lightly oxidised chromium is deposited on thesubstrate (10) from a chromium metal target in an atmosphere of 80% bymass of argon and 20% by mass of oxygen. The thickness of the layer isIn the order of 45 nm.

A barrier layer (30) of SiO₂ is deposited on the first layer from an Simetal target in an atmosphere of 75% by mass of argon and 25% by mass ofoxygen. The thickness of the layer is in the order of 40 nm.

A layer of TiO₂ (40) is deposited on the barrier layer from an oxidisedtitanium target in an atmosphere of 75% by mass of argon and 25% by massof oxygen. The thickness of the layer is in the order of 60 nm.

A last very fine layer of SiO₂ (50) is then deposited on the coatingstack The deposit is performed from an Si metal target in an atmosphereof 75% by mass of argon and 25% by mass of oxygen. The thickness of thelayer is in the order of 5 nm.

The coated substrate Is then subjected to a thermal treatment for 1 hourat 400° C. The rise in temperature occurs rapidly but cooling isconducted very progressively (approximately 3° C. per minute).

The light reflection factor (LR) integrated over the entire visiblerange is measured in accordance with the standard SAE J 964 with anintegrating photometer. The substrate coated according to Example 1 hasa LR of 65%, while the same stack of SiO₂/TiO₂/SiO₂ on a chromium metallayer of the same thickness would have given a LR of 80% and wouldtherefore have given too much glare for use as a rear-view mirror (seeFIG. 3).

The reflected colour of the coating stack is determined by thecolorimetric coordinates L*, a*, b* on the basis of illuminant D65 withan angle of incidence of 2°. The values obtained are collated in thetable below. The very low values for a* and b* show that the coatingstack does not have any significant reflected colour.

The light transmission (LT) integrated over the visible range is 0.9%.

EXAMPLE 2

A coating stack comprising glass/Cr_(x)N_(y)/SiO₂/TiO₂/SiO₂ of bluecolouration, as also shown in FIG. 1, is formed on a clear soda-limeglass (10) with a thickness of 2 mm by cathodic magnetron sputtering.

The conditions of depositing the different layers forming the stack areas follows:

A first layer (20) of lightly nitrided chromium is deposited on thesubstrate from a chromium metal target in an atmosphere of 50% by massof argon and 50% by mass of nitrogen. The thickness of the layer is inthe order of 45 nm.

A barrier layer (30) of SiO₂ with a thickness in the order of 25 nm,then a TiO₂ layer (40) with a thickness in the order of 40 nm, and thena last layer of SiO₂ (50) with a thickness in the order of 5 nm aresuccessively deposited in the same conditions as described in Example 1.

The coated substrate is then subjected to a thermal treatment under thesame conditions as described in Example 1.

The light reflection factor (LR) integrated over the entire visiblerange is measured in accordance with the standard SAE J 964 with anIntegrating photometer. The substrate coated according to Example 2 hasa LR of 43%, while the same coating stack of SiO₂/TiO₂/SiO₂ on achromium metal layer of the same thickness would have given a LR of 56%(see FIG. 4).

The reflected colour of the coating stack is determined by thecolorimetric coordinates L*, a*, b* on the basis of illuminant D65. Thevalues obtained are collated in the table below. The negative values forb* and the very slightly negative values for a* show that the coatingstack has a slightly greenish reflected blue colour.

The light transmission (LT) integrated over the visible range is 1.5%.

EXAMPLE 3

A coating stack comprising glass/Cr_(x)N_(y)/SiO₂/TiO₂/SiO₂ of neutralcolour, as also shown in FIG. 1, is formed on a clear soda-lime glass(10) with a thickness of 2 mm by cathodic magnetron sputtering in thesame conditions as in example 2.

The thickness of the layers are: 75 nm for the Cr_(x)N_(y) layer (20),55 nm for the SiO₂ barrier layer (30), 50 nm for the TiO₂ layer (40) andaround 5 nm for the SiO₂ top layer (50).

The coated substrate is then subjected to a thermal treatment under thesame conditions as described in Example 1.

The level of nitridation of the Cr_(x)N_(y) layer has been analysed. Theindex x is evaluated at 0.7 and y at 0.3.

The substrate coated according to Example 3 has a LR of 68%, while thesame coating stack of SiO₂/TiO₂/SiO₂ on a chromium metal layer of thesame thickness would have given a LR of 76% (see FIG. 5).

The colorimetric coordinates L*, a*, b* of the reflected colour arecollated in the table below. The very low values for a* and b* show thatthe coating stack does not have any significant reflected colour. TABLE1 LR L* a* b* Example 1 65 85.8 −3.8 −1.4 Example 2 43 75.1 −6.4 −16.4Example 3 68 76 −3.73 −2.36

1-19. (canceled)
 20. Composite article comprising a substrate, areflective layer and a titanium dioxide-based photocatalytic layer, saidarticle being characterized in that the reflective layer is composed ofan oxidized or nitrided metal such that the total light reflectionintegrated over the entire visible range of the composite article is inthe range of between 40 and 75%.
 21. Article according to claim 20,characterized in that oxidized or nitrided metal of the reflective layeris in an under-stoichiometric state.
 22. Article according to claim 20,characterized by one of the following: a) the layers are disposed on thesame face of the substrate, or b) the reflective layer is disposed onthe rear face and the photocatalytic layer is disposed on the front faceof the substrate.
 23. Article according to claim 20, characterized by atleast one of the following: a) a barrier layer between thephotocatalytic layer and the substrate, or b) a surface layer on thefront face, or c) the light reflection integrated over the entirevisible range lies between 45% and 70%.
 24. Article according to claim23, characterized by any two of a) or b) or c).
 25. Article according toclaim 23, characterized by all of a) and b) and c).
 26. Articleaccording to claim 20, characterized by at least one of the following(a), (b), (c) or (d). (a) the reflective layer has a thickness (i) inthe range of between 20 and 100 nm or; (ii) in the range of between 30and 60 nm; (b) the photocatalytic layer has a thickness (i) in the rangeof between 20 and 120 nm or; (ii) in the range of between 40 and 75 nm;(c) the article comprises a barrier layer between the photocatalyticlayer and the substrate, the barrier layer having a thickness (i) in therange of between 10 and 80 nm, or; (ii) in the range of between 20 and60 nm; (d) the article comprises a surface layer on the front face, thesurface layer having a thickness (i) in the range of between 2 and 10nm, or; (ii) in the range of between 3 and 6 nm.
 27. Article accordingto claim 26, including at least two of the features (a), (b), (c) or(d).
 28. Article according to claim 26, including all of the features(a), (b), (c) and (d).
 29. Article according to claim 20, characterizedin that the metal of the reflective layer is selected from Cr, Ti, Al,Si, Zr and the alloys of these metals.
 30. Article according to claim20, characterized in that it comprises at least one of (a) a siliconoxide barrier layer between the photocatalytic layer and the substrateor (b) a silicon oxide surface layer on the front face.
 31. Articleaccording to claim 20, characterized by one of the following (a) or (b)(a) in that when the reflected colour is neutral (i.e. when thecoefficients a* and b* of the Lab system are between −5 and 5), thereflection factor lies (i) between 55 and 75%, or (ii) between 60 and72%; (b) in that when the reflected colour is within the blue range,(i.e. when the coefficient a* of the Lab system are between −10 and 0and the coefficient b* of the Lab system is less than −10, thereflection factor lies (i) between 40 and 55%, or (ii) between 40 and50%.
 32. Process for preparing a composite article, characterized inthat it comprises the following steps: depositing a lightly oxidized ornitrided metal layer on one or other of the faces of a support bycathodic magnetron sputtering in a controlled reactive atmosphere;depositing a photocatalytic layer (40) on the front face of the supportby cathodic magnetron sputtering; thermal treating for a period ofbetween 15 minutes to 6 hours (i) at a temperature in the range ofbetween 300 and 500° C., or (ii) at a temperature in the range ofbetween 350 and 450° C.
 33. Process according to claim 32, characterizedin that it comprises a step of depositing a SiO₂ barrier layer bycathodic sputtering prior to depositing the photocatalytic layer. 34.Process according to claim 32, characterized in that it comprises a stepof depositing a fine hydrophilic surface layer by cathodic magnetronsputtering.
 35. Process according to claim 33, characterized in that itcomprises a step of depositing a fine hydrophilic surface layer bycathodic magnetron sputtering.
 36. Use as a rear-view mirror of a motorvehicle of the composite article a) according to claim 20, or b) madeaccording to claim 32.